Porous Conductive Textiles for Wearable Electronics.

Over the years, researchers have made significant strides in the development of novel flexible/stretchable and conductive materials, enabling the creation of cutting-edge electronic devices for wearable applications. Among these, porous conductive textiles (PCTs) have emerged as an ideal material platform for wearable electronics, owing to their light weight, flexibility, permeability, and wearing comfort. This Review aims to present a comprehensive overview of the progress and state of the art of utilizing PCTs for the design and fabrication of a wide variety of wearable electronic devices and their integrated wearable systems. To begin with, we elucidate how PCTs revolutionize the form factors of wearable electronics. We then discuss the preparation strategies of PCTs, in terms of the raw materials, fabrication processes, and key properties. Afterward, we provide detailed illustrations of how PCTs are used as basic building blocks to design and fabricate a wide variety of intrinsically flexible or stretchable devices, including sensors, actuators, therapeutic devices, energy-harvesting and storage devices, and displays. We further describe the techniques and strategies for wearable electronic systems either by hybridizing conventional off-the-shelf rigid electronic components with PCTs or by integrating multiple fibrous devices made of PCTs. Subsequently, we highlight some important wearable application scenarios in healthcare, sports and training, converging technologies, and professional specialists. At the end of the Review, we discuss the challenges and perspectives on future research directions and give overall conclusions. As the demand for more personalized and interconnected devices continues to grow, PCT-based wearables hold immense potential to redefine the landscape of wearable technology and reshape the way we live, work, and play.

Well-defined in-textile photolithography towards permeable textile electronics.

Textile-based wearable electronics have attracted intensive research interest due to their excellent flexibility and breathability inherent in the unique three-dimensional porous structures. However, one of the challenges lies in achieving highly conductive patterns with high precision and robustness without sacrificing the wearing comfort. Herein, we developed a universal and robust in-textile photolithography strategy for precise and uniform metal patterning on porous textile architectures. The as-fabricated metal patterns realized a high precision of sub-100 µm with desirable mechanical stability, washability, and permeability. Moreover, such controllable coating permeated inside the textile scaffold contributes to the significant performance enhancement of miniaturized devices and electronics integration through both sides of the textiles. As a proof-of-concept, a fully integrated in-textiles system for multiplexed sweat sensing was demonstrated. The proposed method opens up new possibilities for constructing multifunctional textile-based flexible electronics with reliable performance and wearing comfort.

Reliability, Validity, and Identification Ability of a Commercialized Waist-Attached Inertial Measurement Unit (IMU) Sensor-Based System in Fall Risk Assessment of Older People.

Falls are a prevalent cause of injury among older people. While some wearable inertial measurement unit (IMU) sensor-based systems have been widely investigated for fall risk assessment, their reliability, validity, and identification ability in community-dwelling older people remain unclear. Therefore, this study evaluated the performance of a commercially available IMU sensor-based fall risk assessment system among 20 community-dwelling older recurrent fallers (with a history of ≥2 falls in the past 12 months) and 20 community-dwelling older non-fallers (no history of falls in the past 12 months), together with applying the clinical scale of the Mini-Balance Evaluation Systems Test (Mini-BESTest). The results show that the IMU sensor-based system exhibited a significant moderate to excellent test-retest reliability (ICC = 0.838, p < 0.001), an acceptable level of internal consistency reliability (Spearman's rho = 0.471, p = 0.002), an acceptable convergent validity (Cronbach's α = 0.712), and an area under the curve (AUC) value of 0.590 for the IMU sensor-based receiver-operating characteristic (ROC) curve. The findings suggest that while the evaluated IMU sensor-based system exhibited good reliability and acceptable validity, it might not be able to fully identify the recurrent fallers and non-fallers in a community-dwelling older population. Further system optimization is still needed.

A monolithically integrated in-textile wristband for wireless epidermal biosensing.

Textile bioelectronics that allow comfortable epidermal contact hold great promise in noninvasive biosensing. However, their applications are limited mainly because of the large intrinsic electrical resistance and low compatibility for electronics integration. We report an integrated wristband that consists of multifunctional modules in a single piece of textile to realize wireless epidermal biosensing. The in-textile metallic patterning and reliable interconnect encapsulation contribute to the excellent electrical conductivity, mechanical robustness, and waterproofness that are competitive with conventional flexible devices. Moreover, the well-maintained porous textile architectures deliver air permeability of 79 mm s-1 and moisture permeability of 270 g m-2 day-1, which are more than one order of magnitude higher than medical tapes, thus ensuring superior wearing comfort. The integrated in-textile wristband performed continuous sweat potassium monitoring in the range of 0.3 to 40 mM with long-term stability, demonstrating its great potential for wearable fitness monitoring and point-of-care testing.

A Review of Structure Engineering of Strain-Tolerant Architectures for Stretchable Electronics.

Stretchable electronics possess significant advantages over their conventional rigid counterparts and boost game-changing applications such as bioelectronics, flexible displays, wearable health monitors, etc. It is, nevertheless, a formidable task to impart stretchability to brittle electronic materials such as silicon. This review provides a concise but critical discussion of the prevailing structural engineering strategies for achieving strain-tolerant electronic devices. Not only the more commonly discussed lateral designs of structures such as island-bridge, wavy structures, fractals, and kirigami, but also the less discussed vertical architectures such as strain isolation and elastoplastic principle are reviewed. Future opportunities are envisaged at the end of the paper.

Wet-Adaptive Electronic Skin.

Skin electronics provides remarkable opportunities for non-invasive and long-term monitoring of a wide variety of biophysical and physiological signals that are closely related to health, medicine, and human-machine interactions. Nevertheless, conventional skin electronics fabricated on elastic thin films are difficult to adapt to the wet microenvironments of the skin: Elastic thin films are non-permeable, which block the skin perspiration; Elastic thin films are difficult to adhere to wet skin; Most skin electronics are difficult to work underwater. Here, a Wet-Adaptive Electronic Skin (WADE-skin) is reported, which consists of a next-to-skin wet-adhesive fibrous layer, a next-to-air waterproof fibrous layer, and a stretchable and permeable liquid metal electrode layer. While the electronic functionality is determined by the electrode design, this WADE-skin simultaneously offers superb stretchability, wet adhesion, permeability, biocompatibility, and waterproof property. The WADE-skin can rapidly adhere to human skin after contact for a few seconds and stably maintain the adhesion over weeks even under wet conditions, without showing any negative effect to the skin health. The use of WADE-skin is demonstrated for the stable recording of electrocardiogram during intensive sweating as well as underwater activities, and as the strain sensor for the underwater operation of virtual reality-mediated human-machine interactions.

Stretchable and Self-Healable Fiber-Shaped Conductors Suitable for Harsh Environments.

Fiber-shaped conductors with high electrical conductivity, stretchability, and durability have attracted increasing attention due to their potential for integration into arbitrary wearable forms. However, these fiber conductors still suffer from low reliability and short life span, particularly in harsh environments. Herein, a conductive, environment-tolerant, stretchable, and healable fiber conductor (CESH), which consists of a self-healable and stretchable organohydrogel fiber core, a conductive and buckled silver nanowire coating, and a self-healable and waterproof protective sheath, is reported. Such a multilayer core-sheath design not only offers high stretchability (≈2400%), high electrical conductivity (1.0 × 106 S m-1 ), outstanding self-healing ability and durability, but also possesses unprecedented tolerance in harsh environments including wide working temperature (-60-20 °C), arid (≈10 % RH (RH: room humidity)), and underwater conditions. As proof-of-concept demonstrations, CESHs are integrated into various wearable formats as interconnectors to steadily perform the electric function under different mechanical deformations and harsh conditions. Such a new type of multifunctional fiber conductors can bridge the gap in stretchable and self-healing fiber technologies by providing ultrastable electrical conductance and excellent environmental tolerance, which can greatly expand the range of applications for fiber conductors.

Wafer-patterned, permeable, and stretchable liquid metal microelectrodes for implantable bioelectronics with chronic biocompatibility.

Implantable bioelectronics provide unprecedented opportunities for real-time and continuous monitoring of physiological signals of living bodies. Most bioelectronics adopt thin-film substrates such as polyimide and polydimethylsiloxane that exhibit high levels of flexibility and stretchability. However, the low permeability and relatively high modulus of these thin films hamper the long-term biocompatibility. In contrast, devices fabricated on porous substrates show the advantages of high permeability but suffer from low patterning density. Here, we report a wafer-scale patternable strategy for the high-resolution fabrication of supersoft, stretchable, and permeable liquid metal microelectrodes (µLMEs). We demonstrate 2-µm patterning capability, or an ultrahigh density of ~75,500 electrodes/cm2, of µLME arrays on a wafer-size (diameter, 100 mm) elastic fiber mat by photolithography. We implant the µLME array as a neural interface for high spatiotemporal mapping and intervention of electrocorticography signals of living rats. The implanted µLMEs have chronic biocompatibility over a period of eight months.

Metallic Glass-Fiber Fabrics: A New Type of Flexible, Super-Lightweight, and 3D Current Collector for Lithium Batteries.

Current collectors are indispensable parts that provide electron transport and mechanical support of electrode materials in a battery. Nowadays, thin metal foils made of Cu and Al are used as current collectors of lithium batteries, but they do not contribute to the storage capacity. Therefore, decreasing the weight of current collectors can directly enhance the energy density of a battery. However, limited by the requirements of mechanical strength, it is difficult to reduce the weight of metal foils any further. Herein, a new type of current collectors made of 3D metallic glass-fiber fabrics (MGFs), which shows advantages of super-lightweight (2.9-3.2 mg cm⁻2 ), outstanding electrochemical stability for cathodes and anodes of lithium-ion and lithium-metal batteries (LMBs), fire resistance, high strength, and flexibility suitable for roll-to-roll electrode fabrication is reported. The gravimetric energy densities of lithium batteries exhibit improvements of 9-18% by only replacing the metal foils with the MGFs. In addition, MGFs are suitable for the fabrication of flexible batteries. A high-energy-density flexible lithium battery with an outstanding figure of merit of flexible battery (fbFOM ) and flexing stability is demonstrated.

Technology Roadmap for Flexible Sensors.

Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

Elasto-Plastic Design of Ultrathin Interlayer for Enhancing Strain Tolerance of Flexible Electronics.

The ability to tolerate large strains during various degrees of deformation is a core issue in the development of flexible electronics. Commonly used strategies nowadays to enhance the strain tolerance of thin film devices focus on the optimization of the device architecture and the increase of bonding at the materials interface. In this paper, we propose a strategy, namely elasto-plastic design of an ultrathin interlayer, to boost the strain tolerance of flexible electronics. We demonstrate that insertion of an ultrathin, stiff (high Young's modulus) and elastic (high yield strain) interlayer between an upper rigid film/device and a soft substrate, regardless of the substrate thickness or the interfacial bonding, can significantly reduce the actual strain applied on the film/device when the substrate is bent. Being independent of existing strategies, the elasto-plastic design strategy offers an effective method to enhance the device flexibility without redesigning the device structure or altering the material interface.

Electrochemical Replication and Transfer for Low-Cost, Sub-100 nm Patterning of Materials on Flexible Substrates.

The fabrication of high-resolution patterns on flexible substrates is an essential step in the development of flexible electronics. However, the patterning process on flexible substrates often requires expensive equipment and tedious lithographic processing. Here, a bottom-up patterning technique, termed electrochemical replication and transfer (ERT) is reported, which fabricates multiscale patterns of a wide variety of materials by selective electrodeposition of target materials on a predefined template, and subsequent transfer of the electrodeposited materials to a flexible substrate, while leaving the undamaged template for reuse for over 100 times. The additive and parallel patterning attribute of ERT allows the fabrication of multiscale patterns with resolutions spanning from sub-100 nm to many centimeters simultaneously, which overcomes the trade-off between resolution and throughput of conventional patterning techniques. ERT is suitable for fabricating a wide variety of materials including metals, semiconductors, metal oxides, and polymers into arbitrary shapes on flexible substrates at a very low cost.

Destructive-Treatment-Free Rapid Polymer-Assisted Metal Deposition for Versatile Electronic Textiles.

Highly conductive, durable, and breathable metal-coated textiles are critical building block materials for future wearable electronics. In order to enhance the metal adhesion on the textile surface, existing solution-based approaches to preparing these materials require time-consuming presynthesis and/or premodification processes, typically in the order of tens of minutes to hours, on textiles prior to metal plating. Herein, we report a UV-induced rapid polymer-assisted metal deposition (r-PAMD) that offers a destructive-treatment-free process to deposit highly conductive metals on a wide variety of textile materials, including cotton, polyester, nylon, Kevlar, glass fiber, and carbon cloth. In comparison to the state of the arts, r-PAMD significantly shortens the modification time to several minutes and is compatible with the roll-to-roll fabrication manner. Moreover, the deposited metals show outstanding adhesion, which withstands rigorous flexing, abrasion, and machine washing tests. We demonstrate that these metal-coated textiles are suitable for applications in two vastly different fields, being wearable and washable sensors, and lithium batteries.

Pathway to Developing Permeable Electronics.

Permeable electronics possess the capability of permeating gas and/or liquid while performing the device functionality when attached to human bodies. The permeability of wearable electronics can not only minimize the thermophysiological disturbance to the human body but also ensure a biocompatible human-device interface for long-term, continuous, and real-time health monitoring. To date, how to simultaneously acquire high permeability and multifunctionality is the major challenge of wearable electronics. Here, a critical discussion on the future development of wearable electronics toward permeability is presented. In this perspective, the critical metrics of permeable electronics are discussed, and the historical evolution of wearable technologies is reviewed with highlights of representative examples. The materials and structural strategies for developing high-performance permeable electronics are then analyzed.

Smoothing the Sodium-Metal Anode with a Self-Regulating Alloy Interface for High-Energy and Sustainable Sodium-Metal Batteries.

Because of the large abundance of sodium (Na) as a source material and the easy fabrication of Na-containing compounds, the sodium (Na) battery is a more environmentally friendly and sustainable technology than the lithium-ion battery (LIB). Na-metal batteries (SMBs) are considered promising to realize a high energy density to overtake the cost effectiveness of LIBs, which is critically important in large-scale applications such as grid energy storage. However, the cycling stability of the Na-metal anode faces significant challenges particularly under high cycling capacities, due to the complex failure models caused by the formation of Na dendrites. Here, a universal surface strategy, based on a self-regulating alloy interface of the current collector, to inhibit the formation of Na dendrites is reported. High-capacity (10 mAh cm-2 ) Na-metal anodes can achieve stable cycling for over 1000 h with a low overpotential of 35 mV. When paired with a high-capacity Na3 V2 (PO4 )2 F3 cathode (7 mAh cm-2 ), the SMB delivers an unprecedented energy density (calculated based on all the cell components) over 200 Wh kg-1 with flooded electrolyte, or over 230 Wh kg-1 with lean electrolyte. The dendrite-free SMB also shows high cycling stability with a capacity retention per cycle over 99.9% and an ultrahigh energy efficiency of 95.8%.

Hyperporous magnetic catalyst foam for highly efficient and stable adsorption and reduction of aqueous organic contaminants.

The facile and low-cost fabrication of free-standing magnetic catalysts with high catalytic efficiency, rapid reaction rate and excellent recoverability has been pursued for various catalysis applications, e.g., treating aqueous organic 4-nitrophenol pollutants. Here, we design and fabricate a free-standing nickel-coated hyperporous polymer foam (Ni-HPF) with adjustable shapes and sizes, hierarchical multiscale porous structures, abundant catalytical interfaces and excellent super-paramagnetic properties. Due to the synergistical effect of abundant binding sites and highly catalytic reduction, the as-prepared Ni-HPF has demonstrated high conversion efficiency (> 90% at extremely low concentration of 7.5 µM) and rapid reaction rate (2.58 × 10-3 s-1) for the reduction of organic 4-nitrophenol. Moreover, the magnetic catalyst also holds excellent recoverability (>80% conversion rate even after 1000 cycles) and good reproducibility (>80% conversion rate after 3 months of storage). As such, this work with novel material design and working principle could provide a wide range of potential applications in water purification, chemical catalysis and energy storage devices.

Pathways of Developing High-Energy-Density Flexible Lithium Batteries.

Flexible lithium-based batteries (FLBs) enable the seamless implementation of power supply to flexible and wearable electronics. They not only enhance the energy capacity by fully utilizing the available space but also revolutionize the form factors of future device design. To date, how to simultaneously acquire high energy density and excellent mechanical flexibility is the major challenge of FLBs. Here, a critical discussion for guiding the future development of FLBs toward high energy density and high flexibility is presented. First, the industrial criteria of FLBs for several desirable applications of flexible and wearable electronics are summarized. Then, strategies to achieve flexibility of FLBs are discussed, with highlights of representative examples. The performance of FLBs is benchmarked with a flexible battery plot. New materials and cell design principles are analyzed to realize high-energy-density and high-flexibility FLBs. Other important aspects of FLBs including materials to improve the cycling stability and safety are also discussed.

Permeable superelastic liquid-metal fibre mat enables biocompatible and monolithic stretchable electronics.

Stretchable electronics find widespread uses in a variety of applications such as wearable electronics, on-skin electronics, soft robotics and bioelectronics. Stretchable electronic devices conventionally built with elastomeric thin films show a lack of permeability, which not only impedes wearing comfort and creates skin inflammation over long-term wearing but also limits the design form factors of device integration in the vertical direction. Here, we report a stretchable conductor that is fabricated by simply coating or printing liquid metal onto an electrospun elastomeric fibre mat. We call this stretchable conductor a liquid-metal fibre mat. Liquid metal hanging among the elastomeric fibres self-organizes into a laterally mesh-like and vertically buckled structure, which offers simultaneously high permeability, stretchability, conductivity and electrical stability. Furthermore, the liquid-metal fibre mat shows good biocompatibility and smart adaptiveness to omnidirectional stretching over 1,800% strain. We demonstrate the use of a liquid-metal fibre mat as a building block to realize highly permeable, multifunctional monolithic stretchable electronics.

V2 O5 Textile Cathodes with High Capacity and Stability for Flexible Lithium-Ion Batteries.

Textile-based energy-storage devices are highly appealing for flexible and wearable electronics. Here, a 3D textile cathode with high loading, which couples hollow multishelled structures (HoMSs) with conductive metallic fabric, is reported for high-performance flexible lithium-ion batteries. V2 O5 HoMSs prepared by sequential templating approach are used as active materials and conductive metallic fabrics are applied as current collectors and flexible substrates. Taking advantage of the desirable structure of V2 O5 HoMSs that effectively buffers the volume expansion and alleviates the stress/strain during repeated Li-insertion/extraction processes, as well as the robust flexible metallic-fabric current collector, the as-prepared fabric devices show excellent electrochemical performance and ultrahigh stability. The capacity retains a high value of 222.4 mA h g-1 at a high mass loading of 2.5 mg cm-2 even after 500 charge/discharge cycles, and no obvious performance degradation is observed after hundreds of cycles of bending and folding. These results indicate that V2 O5 HoMSs/metallic-fabric cathode electrode is promising for highly flexible lithium-ion batteries.

Soft Hybrid Scaffold (SHS) Strategy for Realization of Ultrahigh Energy Density of Wearable Aqueous Supercapacitors.

Future wearable electronics requires safe and high-energy-density supercapacitors (SCs). Commercial SCs making use of organic electrolytes show high energy density, but the flammability of the electrolyte raises serious safety concerns. Aqueous SCs, on the other hand, are very safe, but the energy density is low due to the much narrower voltage window and the difficulty of fabricating thick electrodes. A new materials strategy named soft hybrid scaffold (SHS), which allows easy buildup of ultrathick electrodes made of 3D porous pseudo-material-modified carbon networks, is reported. The carbon network provides excellent mechanical stability and electric conductivity, the hierarchically porous structures ensure rapid ionic transport, and the pseudomaterials enlarge the electrochemical window. Asymmetric aqueous SCs using SHS electrodes show higher energy density than both commercial organic SCs and literature-reported aqueous SCs, with good cycle life and mechanical flexibility. The aqueous SC device is tailorable, waterproof, and fire-retardant, representing a high safety toward practical applications.